This invention relates to control devices for electric motors.
Prior art controls for DC motors have not been comprised of purely digital components although the position logic and velocity compensation logic have been constituted by digital elements. Specifically, such controls include a digital to analog converter which directs signals to a DC drive amplifier. The output of the drive amplifier is, in turn, applied to a threshold comparator, which also has as an input a ramp wave form. In addition, such controls have included further analog circuitry, usually in the form of printed circuit cards, to sense and limit armature current, control amplifier saturation and to insure proper balance between forward and reverse SCR's before motion is called for.
Succinctly stated, existing controls are constituted by a significant amount of analog hardware which is relatively expensive and subject to signal drift. In addition, state-of-the-art controls which have an armature current limiting capability must first sense an overcurrent condition before reducing the firing angle. The problem encountered with such a protective scheme is that arcing or demagnetization can easily occur before corrective action is taken.